This invention relates generally to methods and apparatus for drying moving webs and, more particularly, to methods and apparatus for drying webs comprising infrared dryer units and gas-support units in which gas blowings are directed into web support gaps so as to stabilize the run of the web.
The invention, while being useful in the drying of any type of moving web, is particularly suited to the drying of paper or board webs. For example, a typical application of the invention is in the drying of paper webs in connection with coating or surface-sizing.
Paper webs are conventionally coated either by means of separate coating devices or by means of on-machine devices which are integrated into the paper machines themselves, or by means of surface-sizing devices which operate in the drying section of a paper machine. In the latter case, the web to be coated is passed from the end of a multi-cylinder drying section to a coating device whereupon the web is passed to an intermediate dryer and then to an after-dryer consisting, for example, of a group of drying cylinders. A typical application of the invention is for use as such an intermediate dryer, although it is understood that the invention has applicability in many other environments.
Airborne web dryers are also known in which paper and board webs or the like are dried in a contact free manner. Such airborne web dryers are used, for example, after blade, roll or spread-type paper coating devices to dry the wet coating in a contact free manner.
Conventional airborne web dryers in common use operate using jets or blowings of gas, usually air. This contributes to the fact that conventional airborne web dryers are rather long pieces of equipment that require a large amount of space since the distance over which the air blowings act on the web must be relatively long to provide sufficient drying capacity. Another reason that air-borne web dryers require substantial space is that the density of energy consumed which is useful in drying is relatively low.
Other types of dryers useful in drying moving webs are based on radiation effects, such as infrared radiation dryers. Drying by infrared radiation is advantageous in that infrared radiation has a relatively high energy density which increases as the wavelength of the radiation decreases. However, the use of infrared dryers in drying paper webs has been hampered, for example, by the risk that fire may result as the wavelength of the radiation is decreased and the temperature increases.
Airborne web dryers can essentially be divided into two categories depending upon the type and arrangement of the blow nozzles through which the drying and web-supporting gas, usually air, is directed. The blow nozzles can be essentially divided into two groups, namely, positive-pressure or float nozzles and negative-pressure or foil nozzles.
On the other hand, it is desired to improve conventional infrared dryers by providing contact-free support of the web as it passes through the drying or treatment gap while eliminating or reducing fluttering of the web.